Process for the preparation of a high temperature superconductor

ABSTRACT

A process for preparing a superconductor which is a low anisotropy, high temperature superconductor, includes providing a target in molded form comprised of one of the superconductor or constituent elements of the superconductor, the superconductor having a layered crystal structure, having a superconducting transition temperature, Tc, of 110 K or more, and having a composition expressed by Cu 1−z M z Ae 2 Ca x−1 Cu x O y  where M is at least one member selected from the group consisting of (a) a trivalent ion of Tl, and (b) polyvalent ions of Mo, W, and Re, Ae is at least one of Ba and Sr, x ranges from 1 to 10, and y ranges from 2x+1 to 2x+4, and z ranges from 0&lt;z≦0.5; and forming a film of the superconductor from the target on a substrate by one of sputtering or laser abrasion.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a divisional of U.S. patent application Ser. No.08/548,975 filed Oct. 27, 1995, now U.S. Pat. No. 5,919,735 issued Jul.6, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a high temperature superconductor. Morespecifically, it relates to a superconductor usable at the temperatureof liquid nitrogen or a higher temperature, and a superconductor usableat the temperature of liquid nitrogen or a higher temperature, being lowin anisotropy, and having a high critical current density, and a processfor their preparation.

2. Description of the Prior Art

Known copper oxide superconductors having superconducting transitiontemperatures, Tc's, of 110K or more are Tl and Hg type copper oxidesuperconductors. These copper oxide superconductors have high Tc values,but they contain harmful and rare elements such as Tl or Hg. Thus, theyposed marked problems, such as the necessity for anti-pollutionmeasures, the need for cost reduction, and difficulty in securing suchrare resources.

With those conventional superconductors, let the coherence length in theintrafacial direction of the CuO₂ plane (ab plane) in its crystalstructure be ξ_(ab), the coherence length in a direction perpendicularto the CuO₂ plane (c-axis direction) be ξ_(c), and the anisotropy γ ofthe superconducting properties be expressed as ξ_(ab)/ξ_(c). Then, γ=5for the composition YBa₂Cu₃O₇-y with the lowest anisotropy, and thisvalue was considerably high. Thus, those conventional oxidesuperconductors have had short coherence lengths in the c-axis directionperpendicular to the CuO₂ plane, which, in turn, have made criticalcurrent densities under high temperature, high magnetic field conditionssmall. These facts have been an obstacle to their practical use.

SUMMARY OF THE INVENTION

This invention has been accomplished in the light of the foregoingsituations, and is aimed at resolving the aforementioned harmfulness,high anisotropy of superconducting properties, short coherence lengths,and decreases in current densities under high magnetic field conditions.

In other words, an object of the present invention is to provide a hightemperature superconductor free from harmful elements or low in theircontent, and having a superconducting transition temperature, Tc, of 110K or more.

Another object of the present invention is to provide a high temperaturesuperconductor having, in addition to the above-described advantages, alarge coherence length in the c-axis direction, a high current densityunder a high magnetic field, and low anisotropy.

Still another object of the present invention is to provide a processfor preparing such high temperature superconductors.

To solve the aforementioned problems, the present invention uses anoxide of the alkaline earth metal element Ba, Sr or Ca with copper as amatrix, and adds a monovalent alkali metal element or Cu, Ag or Au, ordivalent Cu, Hg, Pb or Cd, or trivalent Y or lanthanum elements forintroduction of a carrier to form a layered copper oxide, therebyrealizing a high temperature superconductor having a superconductingtransition temperature, Tc, of 110 K or more.

The present invention also uses an oxide of the alkaline earth metalelement Ba, Sr or Ca with copper as a matrix, and adds various elementsfor introduction of a carrier and stabilization of the structure to forma layered copper oxide with as short a c-axis as possible, therebyrealizing a low anisotropy, high temperature superconductor having asuperconducting transition temperature, Tc, of 110 K or more, and lowanisotropy of superconducting properties.

The above-mentioned high temperature superconductor or low anisotropy,high temperature superconductor is prepared by use of a non-equilibriummethod such as high pressure synthesis, hot pressing, HIP (hightemperature isostatic processing), sputtering or laser abrasion. Thetarget in sputtering may be either a sintered material of the samecomposition as a film to be produced, or those targets for therespective elements which are to be laminated for atomic layers.Sputtering or laser abrasion is performed, for example, using an SrTiO₃(100 plane) substrate at a substrate temperature of 300 to 600° C. andan oxygen partial pressure of 0.001 to 1 Torr.

That is, the high temperature superconductor according to the presentinvention is characterized by having a composition expressed as thefollowing formula (1)

MAe₂Ca_(x−1)Cu_(x)O_(y)

where M is at least one member selected from the group consisting ofmonovalent ions of Cu, Ag and Au, divalent ions of Cu, Hg and Pb, andtrivalent Ions of Tl, Bi, Y and lanthanide elements, Hg or Tl does notexist singly, Ae is at least one of Ba and Sr, x=1 to 10, and y=2x to2x+4,

having a layered crystal structure, and having a superconductingtransition temperature, Tc, of 110 K or more.

The high temperature superconductor according to the present inventionis characterized by having a composition expressed as the followingformula (2)

(M,Ae)₃Ae′_(x−1)Cu_(x)O_(y)

where M is at least one member selected from the group consisting ofmonovalent ions of Cu, Ag and Au, divalent ions of Cu, Hg and Pb, andtrivalent ions of Tl, Bi, Y and lanthanide elements, Hg or Tl does notexist singly, Ae is a mixture of Ba and Sr, Ae′ is a mixture of Ba, Srand Ca, x=1 to 10, and y=2x to 2x+4,

having a layered crystal structure, and having a superconductingtransition temperature, Tc, of 110 K or more.

The low anisotropy, high temperature superconductor of the presentinvention is characterized by having a composition expressed as thefollowing formula (3)

Cu_(1−z)M_(z)Ae₂Ca_(x−1)Cu_(x)O_(y)

where M is at least one member selected from the group consisting ofmonovalent ions of Li, Na, K, Rb, Cs, Ag and Au, divalent ions of Hg, Cdand Pb, trivalent ions of Tl, Bi, Y and lanthanide elements, andpolyvalent ions of Cr, Mn, Fe, Co, Ni, Mo, Tc, Ru, Rh, Pd, W, Re, Os, Irand Pt, Ae is at least one of Ba, Sr and Ba_(1−w)Sr_(w), x=1 to 10, andy=2x+1 to 2x+4, z=0 to 0.5, and w=0 to 1.0,

having a layered crystal structure, and having a superconductingtransition temperature, Tc, of 110 K or more, and by being constitutedsuch that when letting ξ_(ab) denote the coherence length in theintrafacial direction of the CuO₂ plane (ab plane) in the crystalstructure, and ξ_(c) the coherence length in a direction perpendicularto the CuO₂ plane (c-axis direction), the anisotropy γ of thesuperconducting properties, expressed as ξ_(ab)/ξ_(c), is 5 or less.

In the formula (3), it is preferred that Ae is Ba, z=0 to 0.3, and x=2to 6. Further preferably, the low anisotropy, high temperaturesuperconductor has a composition expressed substantially asCuBa₂Ca₃Cu₄O_(12−n) where n=0 to 4.

In the formula (3), it is also preferred that M is Re, Ae is Ba, and x=2to 6. Further preferably, the low anisotropy, high temperaturesuperconductor has a composition expressed substantially as(Cu,Re)Ba₂Ca₃Cu₄O_(12−n) where n=0 to 4.

In the formula (3), it is also preferred that M is Ag, Ae is Ba, and x=2to 6. Further preferably, the low anisotropy, high temperaturesuperconductor has a composition expressed substantially as(Cu,Ag)Ba₂Ca₃Cu₄O_(12−n) where n=0 to 4.

In the formula (3), it is also preferred that M is Ag, Ae is Sr, and x=2to 6. Further preferably, the low anisotropy, high temperaturesuperconductor has a composition expressed substantially asAgSr₂Ca₃Cu₄O_(12−n) where n=0 to 4.

The process of the present invention is a process for preparing theabove-described high temperature superconductor or low anisotropy, hightemperature superconductor, which comprises mixing the starting powdersto form a mixture of a desired composition, and subjecting the powdermixture to a high pressure, high temperature synthesis involving apressure of 100 kg/cm² to 100,000 kg/cm², and a temperature of 300 to1,300° C.

In the above process, the starting powders may be powders formed bypulverizing sintered high temperature superconductors that have beenprepared.

The process for preparation of the present invention is alsocharacterized by using a target comprising a molded high temperaturesuperconductor or its constituent elements, and forming a hightemperature superconductor film from the target on a substrate bysputtering.

The process for preparation of the present invention is furthercharacterized by using a molded high temperature superconductor as atarget, and forming a high temperature superconductor film from thetarget on a substrate by laser abrasion.

In the present invention, an oxide of a rare earth element with copperthat can become a superconductor, such as Ba₂Ca_(x−1)Cu_(x)O_(2x+1) orSr₂Ca_(x−1)Cu_(x)O_(2x+1), is used as a matrix, and a carrier isintroduced using a mono- to trivalent element to realize a hightemperature superconductor. By making the number of copper layers large,its superconducting transition temperature Tc can be raised. Also bymaking the layer of monovalent ions, the layer of divalent or trivalentions, or the layer of mono- to trivalent ions mixed a single layer, andmaking this layer a defect layer with many vacancies, it becomespossible to decrease the interlayer distance of the copper layers andincrease the critical current density.

In the present invention, moreover, Cu_(1−z)Ba₂Ca_(x−1)Cu_(x)O_(2x+4−n),Cu_(1−z)Sr₂Ca_(x−1)Cu_(x)O_(2x+4−n), orCu_(1−z)(Ba_(1−w)Sr_(w))₂Ca_(x−1)Cu_(x)O_(2x+4−n), a copper-based hightemperature superconductor having a short c-axis and apt to formvacancies of Cu, is used as a matrix. In the composition indicated, w isa numerical value of 0-1 in the desired composition shown in theaforementioned formula (3), but it is a slightly larger value. As willbe mentioned later, ions such as Cu ions are added to any of thosematrices to decrease vacancies of Cu and O, perform stabilization of thestructure and optimization of the carrier concentration, and increasethe superconducting bonds between the unit lattices, thereby realizing alow anisotropy, high temperature superconductor. Also, the number (x) oflayers of copper and the carrier concentration are optimized to raisethe superconducting transition temperature Tc. Furthermore, the CuOlayer between the adjacent Ba—O layers is rendered superconductive toreinforce the bonds between the Ca_(x−1)Cu_(x)O_(2x) superconductingblocks, reduce the anisotropy of superconductivity and increase thecoherence length in the c-axis direction, thereby increasing thecritical current density under a magnetic field.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the X-ray diffraction pattern of an Ag-1234sample, AgBa₂Ca₃Cu₄O_(12−n) (n=0 to 4), an embodiment of a hightemperature superconductor according to the present invention;

FIG. 2 is a characteristic graph showing the temperature dependence ofthe magnetic susceptibility of she Ag-1234 sample;

FIG. 3 is a characteristic graph showing the temperature dependence ofthe electrical resistance of the Ag-1234 sample;

FIG. 4 is a graph showing the X-ray diffraction pattern of a Cu-1234sample, CuBa₂Ca₃Cu₄O_(12−n) (n=0 to 4), an embodiment of a hightemperature superconductor according to the present invention;

FIG. 5 is a characteristic graph showing the temperature dependence ofthe magnetic susceptibility of she Cu-1234 sample;

FIG. 6 is a characteristic graph showing the temperature dependence ofthe electrical resistance of the Cu-1234 sample;

FIG. 7 is a characteristic graph showing the temperature dependence ofthe superconducting properties of the Cu-1234 sample; and

FIG. 8 is a characteristic graph showing the temperature dependence ofthe superconducting properties of an (Ag,Cu)-1234 sample.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following examples are described preferred embodiments of thepresent invention. However, these embodiments are intended to illustratethe invention, and are not to be construed to limit it.

EXAMPLE 1

Sintered samples of the composition Ba₂Ca_(x−1)Cu_(x)O_(2x+1) of theaforementioned formula (1) but excluding M were prepared with x beingvaried in the range of from 3 to 6. AgO or Ag₂O powder was added to formcharges of the composition AgBa₂Ca_(x−1)Cu_(x)O_(2x+1+r) (r=0.5 or 1).These charges were each sealed in an Au or Pt capsule, and treated for0.5 to 5 hours at a high pressure of 1 to 6 GPa and a high temperatureof 800 to 1,200° C. As a result, superconductors with superconductingtransition temperatures, Tc, of 110 K or more were obtained.

The X-ray diffraction pattern of a sample of the resulting substanceswas obtained for identification, and is shown in FIG. 1. This is asample with x=4. The composition of the resulting superconductor wasidentified as (Ag,Cu)Ba₂Ca₃Cu₄O_(12−n) (n=0 to 4). This sample isdesignated as Ag-1234. This designation is done in order to distinguishthis sample from Cu-1234 sample which contains no Ag. In identifying thecomposition, the space group represented by the Hermann-Mauguin's symbolwas approximatively expressed as P4/mmm. As the X-ray diffractionpattern shows, the number of copper layers of this superconductor sampleis 4. FIG. 2 is a graph showing the temperature dependence of themagnetic susceptibility χ of the Ag-1234 superconductor. As illustratedthere, the magnetic susceptibility χ begins to decrease at 117 K. FIG. 3shows the temperature dependence of the electrical resistance of thissample. The measurements of the electrical resistance also show thesuperconducting transition temperature Tc=117 K.

EXAMPLE 2

Sintered samples of the composition Sr₂Ca_(x−1)Cu_(x)O_(2x+1) wereprepared with x being varied in the range of from 3 to 6. AgO or Ag₂Opowder was added to form charges of as similar a composition as possibleto the composition AgSr₂Ca_(x−1)Cu_(x)O_(2x+1+r) (r=0.5 or 1). As inEXAMPLE 1, these charges were each sealed in an Au or Pt capsule, andtreated for 0.5 to 5 hours at a high pressure of 1 to 6 GPa and a hightemperature of 800 to 1,200° C. As a result, superconductors withsuperconducting transition temperatures, To, of 110 K or more wereobtained.

EXAMPLE 3

Powdery samples of the composition Ba₂Ca_(x−1)Cu_(x)O_(2x−1) and thecomposition Ba₂Ca_(x−1)Cu_(x+1)O_(2x+2) were prepared with x beingadjusted in the range of from 2 to 6. To each of the samples was addedCaO₂ or BaO₂ powder so that the average valence of Cu would be 2.3,thereby forming charges of as similar a composition as possible to thecomposition CuBa₂Ca_(x−1)Cu_(x)O_(12−n) (n=0 to 4). As in EXAMPLE 1,these charges were each sealed in an Au or Pt capsule, and treated for0.5 to 5 hours at a high pressure of 1 to 6 GPa and a high temperatureof 800 to 1,200° C. As a result, superconductors with superconductingtransition temperatures, Tc, of 110 K or more were obtained.

The X-ray diffraction pattern of a sample of the resulting substanceswas obtained for identification, and is shown in FIG. 4. Based on thisX-ray diffraction pattern, the composition of the resultingsuperconductors was identified as CuBa₂Ca₃Cu₄O_(12−n) (n=0 to 4). Thissample is designated as Cu-1234. In identifying the composition, thespace group represented by the Hermann-Mauguin's symbol wasapproximatively expressed as P4/mmm. FIG. 5 is a graph showing thetemperature dependence of the magnetic susceptibility χ of the Cu-1234superconductor. As illustrated there, the magnetic susceptibility χbegins to decrease at 116 K. FIG. 6 shows the temperature dependence ofthe electrical resistance of this sample. The measurements of theelectrical resistance also show the superconducting transitiontemperature Tc=116 K or more.

The anisotropy of the superconducting properties of the thus obtainedCuBa₂Ca₃Cu₄O_(12−n) sample was γ=1.3 to 2.5, the lowest value comparedwith those of conventional copper oxide superconductors. FIG. 7 showsthe temperature dependence of the anisotropy γ of this sample. In thedrawing, data under cooling in a magnetic field (FC) and under coolingin zero magnetic field (ZFC) are revealed. At 100 K or less, especially,at 80 K or less, the anisotropy γ is close to 2. The coherence length inthe c-axis direction of this sample was about 10 Å, the largest lengthcompared with those of conventional copper oxide superconductors. Thecritical current density Jc under a high magnetic field was as nigh as10⁵ A/cm² at a magnetic field of 1 T and a temperature of 77 K.

EXAMPLE 4

ReO₃ or Re₂O₇ was added to the constitution of EXAMPLE 3 to enable thetreating temperature to be lowered. Charges of the composition(Cu_(1−z)Re_(z))Ba₂Ca₃Cu₄O_(12−n) (z=0 to 0.5) were used for productionin the same way as in EXAMPLE 1. Low anisotropy, high temperaturesuperconductors with a superconducting transition temperature, Tc, of116 K and anisotropy γ of 3 to 4 were obtained.

EXAMPLE 5

To powered samples of the composition Sr₂Ca_(x−1)Cu_(x+1)O_(2x+2) (x=2to 6) was added AgO or Ag₂O powder to form samples of the chargingcomposition (Ag,Cu)Sr₂Ca_(x−1)Cu_(x)O_(2x+4−n) (n=0 to 4). As in EXAMPLE1, these samples were sealed in an Au or Pt capsule, and treated for 0.5to 5 hours at a high pressure of 1 to 6 GPa and a high temperature of800 to 1,200° C. As a result, superconductors with superconductingtransition temperatures, Tc, of 110 K or more were obtained.

EXAMPLE 6

When AgO or Ag₂O powder was added in the constitution of EXAMPLE 3,there were obtained low anisotropy, high temperature superconductors ofthe composition (Ag_(1−z)Cu_(z))Sr₂Ca₃Cu₄O_(12−n) (n−0 to 4) having asuperconducting transition temperature Tc=117 K and anisotropy ofsuperconducting properties γ=up to 3. FIG. 8 shows the temperaturedependence of the anisotropy of the resulting sample, (Ag,Cu)-1234.

As described above, the present invention can provide a superconductorhaving a superconducting transition temperature of 110 K or more andfree from Tl or Hg or having a low Tl or Hg content, a superconductorthat has been unfeasible.

The present invention can also provide a superconductor having a highcritical current density, Jc, at a high magnetic field, low anisotropyof superconducting properties, and a superconducting transitiontemperature of 110 K or more, a superconductor that has beennonexistent.

The present invention has been described in detail with respect to thepreferred embodiments, and it will now be clear that changes andmodifications may be made without departing from the invention in itsbroader aspects. It is our intention, therefore, in the appended claimsto cover all such changes and modifications as fall within the truespirit of the invention.

What is claimed is:
 1. A process for preparing a superconductor which isa low anisotropy, high temperature superconductor, comprising: providinga target in molded form comprised of one of the superconductor orconstituent elements of the superconductor, the superconductor having alayered crystal structure, having a superconducting transitiontemperature, Tc, of 110 K or more, and having a composition expressedby: Cu_(1−z)M_(z)Ae₂Ca_(x−1)Cu_(x)O_(y), where M is at least one memberselected from the group consisting of (a) a trivalent ion of Tl, and (b)polyvalent ions of Mo, W, and Re, Ae is at least one of Ba and Sr, xranges from 1 to 10, and y ranges from 2x+1 to 2x+4, and z ranges from0<z≦0.5; and forming a film of the superconductor from the target on asubstrate by one of sputtering or laser abrasion.
 2. The processaccording to claim 1, wherein the film is formed by sputtering, whereinthe layered crystal structure has a CuO₂ plane (ab plane) having anintrafacial direction, and wherein the superconductor has ananisotrophy, γ, of superconducting properties expressed as ξ_(ab)/ξ_(c)which is 5 or less, where ξ_(ab) denotes a coherence length in theintrafacial direction of the CuO₂ plane in the layered crystalstructure, and where ξ_(c) denotes a coherence length in a directionperpendicular to the CuO₂ plane (c-axis direction).
 3. The processaccording to claim 1, wherein the film is formed by laser abrasion,wherein the layered crystal structure has a CuO₂ plane having anintrafacial direction, and wherein the superconductor has ananisotrophy, γ, of superconducting properties expressed as ξ_(ab)/ξ_(c)which is 5 or less, where ξ_(ab) denotes a coherence length in theintrafacial direction of the CuO₂ plane (ab plane) in the layeredcrystal structure, and where ξ_(c) denotes a coherence length in adirection perpendicular to the CuO₂ plane (c-axis direction).